In drilling operations for oil and gas wells, a well bore is formed by drilling through geographic strata. In the drilling process, a pipe casing is positioned within the well bore. The region or space between the outside diameter of the pipe casing and the inner wall of the well bore is termed the annulus. As the depth of the well increases, increasing pressures make it necessary to use smaller bores and smaller pipe casings, resulting in a progressive decrease in the size of the annulus with the depth of the well bore. Sections of well bores that have a small annulus, such as 1/2 to 3/4 of an inch, are commonly referred to as "close tolerance conditions."
Unique problems are encountered when working in close tolerance conditions, including problems with cleaning the well bore prior to carrying out cementing operations. Three basic types of cementing operations are used in the drilling and completion of oil and gas wells: (1) casing or primary cementing; (2) squeeze cementing; and (3) setting of cement plugs.
Casing or primary cementing is used to seal the annulus between the pipe casing and bore hole so that fluids in permeable layers of the surrounding strata may not migrate vertically within the annulus. This procedure involves pumping a desired volume of cement slurry down the casing and back up the annular space a required distance toward the surface. It has been demonstrated and documented in both laboratory testing and field operations that rotation of the pipe casing during primary cementing is one of the most important factors in obtaining successful primary cement jobs.
Squeeze cementing is used when "channeling" or partial bypassing of the cement slurry past the drilling mud media in the well bore results in primary cement jobs of poor quality. If the primary cementing of the casing was inadequate in sealing or supporting the casing string, it is necessary to employ squeeze cementing. Squeeze cementing involves pumping of cement slurry under very high pressure through holes perforated in the pipe casing at precise levels. Squeeze cementing can reduce the oil or gas producing capability of a well due to filling of the permeable section of the reservoir rock with drilling mud and cement. Squeeze cementing is also very costly from the standpoint of the additional time and expense of the extra operations required.
The setting of cement plugs is the third type of cementing operation sometimes required in the drilling of oil and gas wells. Setting of cement plugs consists of filling up the drilled well bore over a specified interval, usually ranging from a few hundred feet to a few thousand feet or more in length with cement in order to create a cement plug.
A cement plug can be used to abandon a well found to be dry or depleted of oil or gas, to change the direction of drilling in order to drill around a section of drill pipe or casing that has become "stuck" and prevented deeper drilling, or to direct the drilling of the well bore in a different direction to a more favorable subsurface position in order to find the oil or gas reservoir. In setting cement plugs, it is desirable to do so with pipe smaller than the drill pipe, since its removal, after the cementing slurry has been pumped into place, does less to disrupt the ability of the cement to form a solid barrier in the well bore.
The number and size of pipe casing strings required for a well depends on the planned depth of the well and the pressures anticipated at the various subsurface levels in the area. Typically, deeper wells require a number of concentric strings of casing. The concentric strings of casing are run and cemented in progressively smaller hole sizes to line the newly drilled hole interval to contain the ever increasing pressures which normally occur in deeper rock layers. For example, a bit which is used to start the well at the surface may be 26 inches in diameter. The 26 inch bit is used to drill a hole or well bore of 26 inches in diameter down to the first casing setting level, where a 20 inch outside diameter (O.D.) casing is installed and cemented. A drill bit of 171/2 inches in diameter would then be used to drill a 171/2 inch diameter well bore down to the next required level, where a 133/8 inch O.D. casing is installed and cemented. The next bit would be 121/4 inches in diameter to drill down to the next critical level, where a 95/8 inch O.D. casing is installed and cemented.
The next bit size would be 81/2 inches in diameter to drill down to the next casing level, where a 75/8 inches O.D. casing is run and cemented, usually as a "liner," which is defined as a short casing string which covers only the open hole interval plus a small overlap into the bottom of the previous casing to save the expense of running the entire length of the 75/8 inch O.D. casing back to the surface. A 61/2 inch bit is then used to drill that size hole below the 75/8 inch casing liner down to the next critical level, where a 51/2 inch O.D. casing could also be run as a "liner."
If the objective zone has still not been reached, a 45/8 inch bit is used to drill below the 51/2 inch O.D. casing liner in preparation for a well completion in a still smaller casing size. From this example, it is evident that the annular space available between the bore hole and the casing or casing liner in deeper wells is very limited and a cleaning tool which can be used safely to aid in the successful primary cementation of casing under such limited clearance conditions is greatly needed in the oil and gas industry to improve the quality of well completions while reducing their ultimate cost.
Whether cementing casing or setting cement plugs, it is desirable for the walls of the well bore to be mechanically cleaned of uncirculatable mud media, sometimes called "mud cake." The formation of mud cake typically results from the loss of drilling fluid when circulating drilling mud in porous strata, such as porous rock, sandstone, limestone, or shale, during drilling operations. A portion of the fluid component of the drilling mud leaks into the surrounding strata, leaving behind a residue of drilling mud on the wall of the well bore, and thus forming the mud cake. Mud cakes can be cleaned from the well bore using abrading devices of cable or wire, known as "scratchers."
Cleaning the well bore prior to cementing operations prevents contamination of the cement slurry by the chemically treated drilling mud and permits better bonding of the cement to the cleaned bore hole. A number of such devices are discussed in the inventor's U.S. Pat. No. 4,750,558, which discussions are incorporated herein by reference.
In the inventor's U.S. Pat. No. 4,750,558, the inventor disclosed and claimed a well bore cleaning tool having a plurality of collars which could be slipped onto a pipe casing without welding, and then secured to the pipe casing using any conventional securing means, such as set screws. A cable was secured to the collars. The cable was disposed longitudinally on one side of the casing, substantially in a straight line. In a preferred embodiment, the tubes were secured to the collars using a series of tubes. The tubes had an inside diameter just slightly greater than the outside diameter of the cable.
The tubes in the '558 patent were spaced at approximately uniform intervals along the cable, and then crimped into an L-shape to secure them to the cable. The L-shaped tubes were placed in L-shaped cut-outs or cavities formed in the collars, and then tack welded into place. The collars were then attached to the casing using set screws. The distance between each successive collar was selected so that the cable would extend a sufficient distance outwardly from the casing to scratch the wall of the bore. For example, if the device was to be used to clean a large diameter bore, the collars were set closer together along the pipe casing, thus increasing the slack in the cable. Likewise, to clean a small diameter bore, the collars were set farther apart along the pipe casing, thus decreasing the slack in the cable.
Although the invention of U.S. Pat. No. 4,750,558 was successful in cleaning well bores; in close tolerance conditions, it was noted that in some very close tolerance conditions, it was necessary to place the cable even closer to the drill pipe than was previously possible. Additionally, in the invention of U.S. Pat. No. 4,750,558, the collars were placed such that the open ends of the L-shaped tubes were on a leading edge of the casing during rotation of the casing. This orientation facilitates frictional contact of the cable with the wall of the well bore. However, it was discovered that in some drilling conditions and applications, this orientation did not produce optimal results.
In view of the problems and shortcomings encountered with the arrangement of the foregoing designs, there is a need for an invention that overcomes the aforementioned problems and shortcomings.